Sleep scientist here. Actigrams based on 3way accelerometers are very common to measure sleep in a very non invasive way and they give a resolution good enough for having an idea of sleep amount and even roughly sleep pattern. I have no idea how well wakemate works, I would love to try of course (wink).

The research-grade equivalent of wakemate is called actiwatch ( http://actiwatch.respironics.com/ ) and it costs a lot more.